Battery and Solar powered heater

Hi, I'm a business student working on a business concept, and I need some help with an electrical problem.

I am attempting to design a product that stays warm, 50 degrees when the temperature outside is 0 or more. This is insulated, have a volume of around 1200 inches, and is powered by the sun and a battery when necessary.

I believe that this can be done using a low voltage heater similar to an electric blanket. One of the electrical engineering professors on campus told me that one way to calculate the wattage necessary to run a heater of this type was to look at the dimensions of an electric blanket, and divide the number of watts needed to power the blanket by the area of the blanket. Using this formula, I calculate that the average power to run a square inch of blanket was .03 watts.

From what I have found in my research, a typical solar cell produces .5 volts at 4 or 5 amps. I understand that you can figure out the wattage by multiplying volts by amperes. Using this formula, I figured that the typical cell outputs 2 to 2.5 watts.

What I am trying to figure out, is the best way to produce heat using these solar cells. My initial thought is to use a series of resisters to produce heat. I also need a way to keep these heated when the sun is not out so that is why I decided to include a battery.
I have the following questions regarding this project.

1. Are my calculations correct?
2. Is this a possibility, or is the power required too great for a battery to produce?
3. If it is possible, how can I calculate the power requirements to keep this warm at the minimum temperature?
4. How can I figure out the life of a battery when used in this situation?

Any help would be greatly appreciated, if you need any more information please let me know. Thanks in advance

Ok, your current from solar cells is pretty high but regardless the wattage is within reason and that's all that matters. I'll come back to solar in a bit.

First, you should establish the heat insulation/transfer value of the storage unit and find the power requirements to maintain the temperature difference. So if you could make a freezer go to 0 you could put a thermometer inside the unit and check to make sure the wattage you believe will work will maintain the temperature. You could always over size the heating portion of the device and regulate it back to maintain the temperature too, and while a much better solution it would be more involved.

Second, solar cells are rated at perfect conditions. Unless on a mountain top, being near the equator generally isn't where you find freezing temps. At a northern latitude where cold conditions occur, they occur because the sunlight is not very effective at heating the ground. The other half of sunlight is where the solar cell functions, and that light is similarily not very effective either. It might be 30%, but it could be as bad as 10% even during the peak time of noon-3pm.

So here goes on your questions:
1) Close enough to begin testing, but you need to know the conditions of the calculations - batteries loose power when really cold too (but they heat up as you use them and will offset this a little)

2) How big of a battery? Like an AA cell or a D cell? Batteries are rated by ampere-hour (Ah or by a 1/1000 unit of mAh) and you could multiply by the voltage to get W-hr or for a AA cell of 2000mAh * 1.5V = 3 W-hr which would seem to be 3W for one hour. But this is the total over 20 hours, if you use it for only an hour you'll only get 1/3 of that or less because most batteries like to be discharged slowly, not fast. Some of the newer NiMH or LiOn batteries could come close to meeting the full power because they are designed to discharge quickly and not suffer the penalty.

3) You need to know the heat transfer characteristics of the storage unit. You could likely make an educated guess based on the insulation (say 1/2" of styrofoam) but the container design matters too, the lid might loose 3x the heat the rest of the unit looses because of how its attached. Likely easiest to test in a freezer unless your engineering students are looking for a neat finite-element-analysis (FEA) project with a computer simulation.

4) What battery type? If a LiOn battery, its going to be pretty close to the W-hr rating if published. You'd really need to know the Peukert factor to properly know the life of the battery. You could probably google for empircal test results with batteries of a similar type and get an idea how long they'd last at whatever current draw. In short though, 2 batteries sharing the load together is going to outlast 1 battery that's switched out for another battery after it. With the new battery technologies, its just not as bad as older technologies.

Thanks for the quick reply. I have some more information, and more questions.

I stuck an electric warming pad that was rated at 46 watts into a container in a 0-degree freezer. The pad managed to keep the heat loss down in the non-insulated container to about 1 degree every 15 – 20 minutes. I’m not certain, and I’m thinking this is something that I would actually have to test, but if I was to cut the heat loss in half with insulation I should be able to maintain the temperature with half that power. ???

If this is the case, then I’m looking at needing to provide a heating element with 23 watts of power. I was looking for batteries online, and found this lithium Ion battery. (3.6V, rated at 2200 mAh. I know most of these numbers will change depending on temperature and other variables, but I just want to make sure I have a feel for the calculations.

I calculated that if I was to wire 10 of these batteries, half in parallel, and half in series, I should get a battery pack that will output a voltage of 18 volts(5 * 3.7 volts), and a capacity of 11000 (5 * 2200 mAh). Using these calculations, I estimated a discharge rat of 1111 ma (20 watts/18 volts =1111 mAh) and a life of 9.9 Hours (11000 / 1111). I’m hoping those calculations are correct.

One thing I still am not sure on, I understand that to recharge a battery, you need to apply a specific voltage, but is the calculation to determine the charge rate the same as the discharge rate?

I don’t need 100% exact numbers, just something that will be close enough to get started, I’d hire an engineer to get it figured out perfectly :). Thanks again for your help.

23 watts is still a lot of power! I'd think a more efficient container design to reduce the heat loss would be the area to work on and not the electrical part.

Ok, batteries in series do add voltages, so 5*3.6V=18V and now you'll have two of those battery packs. Each pack is only good for 2200mAh. With two in parallel, 2*2200mAh = 4400mAh.

Well, regular lead-acid or NiCad batteries can be charged by just applying a higher voltage and its charge rate is going to be similar to the discharge rate and again the Peukert factor rears its ugly head. For the LiOn batteries though, a current-source charging method is used. In effect the charger will have a circuit that will adjust the voltage to feed in the correct amount of current. It will also measure the effective resistance the batteries provide to that current to determine charge level to make sure it doesn't overcharge too. This can be done with an IC chip that costs just a few dollars, and can be much much faster than the old method.

I don't know the current figures, but a while back the mobile Pentiums took around 20W at full speed, and you know how laptop batteries are almost always a dissappointment when it comes to longevity. The handful of laptops where you can still get 2 hours use after a year seems to be more an exception than the norm.

Ah, that will change things quite a bit, did not quite understand the math on the wiring things in parallel. Thanks.

Yeah it seems that making the container that's efficient is actually going to be the major problem. We'll se how that goes. Using that same pack I figured that I would need to cut the wattage needed down to eight watts to get ten hours of charge.

And of course the Peukert factor (which I still need to research) is in effect, but that's something else to deal with.

Thanks a lot Cliff_J, you've helped me a lot, not sure if I'm going to have any more questions, but we'll see. I'm really enjoying reading the forum, most of the conversations are quite a bit out of my league, but I do enjoy reading them. You seem to have a great community here.

I am in an industry which could really benifit from a product like this. I am in the materials testing industry. We take samples of fresh concrete in plastic molds. We then place them in a cure box for 24 hours and leave them on-site. During this 24-hr period the temperature must be maintained at 60-80 degrees F. The concrete does generate it own heat but will generally only keep the temperatures between 40-45 degrees F. We have tried several methods to keep the samples warm. Such as hot water, and hand warmers. Several hand warmers do the job but we would like to find something that is re-usable. There are thousands of companies just like mine that need something like this. If you develop something I would be very interested in buying your product and know a market where it could be employed.

I have never been able to find a battery powered heater. Does someone make one?
Cooling would be nice also, but normally we can regulate the summer temperatures by putting ice in the cure box. Although it can be somewhat of a pain to find ice on remote jobsites. If you have a product that could meet my needs I could use alot of them.

Staff: Mentor

I tried googling battery powered heater to see what I'd find, but the first few hits were for portable things for heating water (like for portable showers). They tended to mention using propane as the fuel source, rather than battery power, so I googled propane heater, and got a bzillion hits. Here's a typical one:

http://www.heatershop.com/propane_heaters.html [Broken]

Would propane-powered heaters work for you? You'd need to be sure that the curing boxes had enough oxygen ventillation, but some of these heaters may even have a thermostat (and battery fan maybe).

I have looked at propane but have some concerns. The cure box is just an igloo cooler and propane would probably melt them. Also they would be left un-attended for 24-hrs. Propane makes me nervous because if any one were to get burned I may be in for a law suit. I guess I am not totally opposed to using a fuel source but right now I am uncomfortable with the options I have seen.